Electromagnetic fields (EM fields) are measured by devices which detect either the electric field (E-field) or magnetic field (H-field) components of the electromagnetic field. Conventionally, electric dipole antennas are used to detect E-fields and magnetic loops are used to detect H-fields.
In a transverse EM field, such as free space, the relationship between E- and H- fields is fixed and is independent of frequency. Under such conditions, one type of sensor may be substituted for another to determine the strength of both fields, as well as the power density. However, in the vicinity close to the source of radiation or a reflecting surface, the "near-field" region, the relationship is not fixed.
The extent of the near-field region depends on the frequency of the electromagnetic waves. At frequencies above 1000 MHz, the region is short and for all practical purposes insignificant. At lower frequencies, where the near-field region may extend from several centimeters to hundreds of meters, measurements of both E-field and H-field components are often required.
Sensing devices may be used in close proximity to a human body, such as personal dosimeters worn on a pocket or belt. In such cases, only the H-field component can be measured reliably, since the E-field can induce surface currents in the body, which result in both absorption as well as the reflection of the rf energy.
E-field probes possess an inherent H-field discrimination, since the induced currents in the dipole antenna are due to the E-field only. In H-field magnetic loop detection, however, both H-field and E-field induced currents flow within the enclosed loop. This results in undesirable errors in measuring the H-field, since the combined rf current is dependent on the E-field polarization.
A common technique for solving the problem of undesirable E-field induced currents in H-field detectors, as disclosed in U.S. Pat. No. 4,023,093, is to shield the magnetic loops. Theoretically, E-field induced currents cannot exist within a completely shielded loop. However, in order to measure the loop current, the shield integrity must be compromised. In practice, maintaining an effective shield over a wide frequency band is problematic.